Wet strength resins are often added to paper and paperboard at the time of manufacture to enhance the strength of these products when wet. Paper manufactured with wet strength resins generally retains at least 10 to 50 percent of its strength when wet. In contrast, paper manufactured without such resins normally retains only 3 to 5 percent of its strength after wetting. Paper having enhanced wet strength is useful in a wide variety of applications, such as toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
Wet strength resins also enhance dry strength in paper. Dry strength is a critical paper property, particularly in light of the recent trend for paper manufacturers to use high yield wood pulps in paper in order to achieve lower costs. These high yield wood pulps generally yield paper with significantly reduced dry strength when compared to paper made from highly refined pulps.
Resins similar to those used for enhancing strength in paper are also often used as creping adhesives. In the manufacture of paper products such as facial tissue, bathroom tissue, or paper towels, the paper web is conventionally subjected to a creping process in order to give it desirable textural characteristics, such as softness and bulk. The creping process typically involves adhering a web (a cellulose web in the case of paper) to a rotating creping cylinder, such as a Yankee dryer, and then dislodging the adhered web with a doctor blade. The impact of the web against the doctor blade ruptures some of the fiber-to-fiber bonds within the web and causes the web to wrinkle or pucker.
The severity of this rupture is dependent upon a number of factors, including the degree of adhesion between the web and the surface of the creping cylinder. Greater adhesion results in increased softness, although generally with some loss of strength. In order to increase adhesion, a creping adhesive may be used to enhance any naturally occurring adhesion that the web may have due to its water content, which will vary widely depending on the extent to which the web has been previously dried.
A desirable creping adhesive is one that adheres the sheet just tightly enough to the drum to give a good crepe, while imparting absorbance and softness with the least possible loss of paper strength. If adhesion to the dryer drum is too strong, the sheet may pick or even “plug” (i.e., underride) the doctor blade, and wrap around the dryer drum. If there is insufficient adhesion, the sheet will lift off too easily and undergo too little creping. In addition to controlling the extent of creping, creping adhesives should also prevent wear of the dryer surface, provide lubrication between the doctor blade and the dryer surface, and reduce chemical corrosion.
The resins can also be used as adhesives or curing agents for adhesive formulations used in manufacturing engineered wood products, such as particleboard, oriented strand board (OSB), waferboard, fiberboard (including medium-density and high-density fiberboard), parallel strand lumber (PSL), laminated strand lumber (LSL), and similar products. Such adhesive compositions may be also be used to produce plywood or laminated veneer lumber (LVL). The engineered wood products can also be described as lignocellulosic-based composites, which are based on smaller wood particles bound together by an adhesive. Application of these adhesives when making engineered wood products and other types of useful materials can be achieved by roller coating, knife coating, extrusion, curtain coating, foam coaters, and spray coaters, for example, a spinning disk resin applicator. Resins are combined with a protein source, such as soy flour or soy protein isolates to form an adhesive. Functionalized polyamine-epichlorohydrin resins typically contain both azetidinium and aminochlorohydrin functionalities and these functional groups react with available amines, alcohols, and carboxylic acids in the adhesive/lignocellulosic system when heated.
Polyamine-epihalohydrin resins, such as polyaminopolyamide-epihalohydrin (PAE) resins, are commonly employed in the paper manufacturing industry as wet-strength resins, dry strength resins, and creping adhesives. Such resins often contain large quantities of epihalohydrin hydrolysis products and inorganic chloride, which is undesirable for workplace safety and environmental reasons. For example, commercial polyaminopolyamide-epichlorohydrin resins typically contain 0.5 to 10 percent by weight (dry basis) of the epichlorohydrin by-products 1,3-dichloropropanol (1,3-DCP), 2,3-dichloropropanol (2,3-DCP), and 3-chloropropanediol (CPD). Production of such resins with reduced levels of epichlorohydrin by-products has been the subject of much investigation. Workplace safety and environmental pressures have been increasing to produce resins with lower levels of epichlorohydrin by-products and other adsorbable organic halogen (AOX) species. AOX is the adsorbable organic halogen content of the resin, which can be determined by means of adsorption onto carbon. AOX species include epichlorohydrin and its by-products 1,3-DCP, 2,3-DCP, and CPD, as well as organic halogen bound to the resin polymer backbone. Removal of inorganic chloride (e.g., chloride salts) reduces the corrosivity of polyamine-epihalohydrin resins and minimizes loss of desired reactive functionality (i.e., the azetidinium moieties on the resin polymer backbone).
Several methods have been devised for reducing the quantities of AOX species in polyamine-epihalohydrin resins in the course of their manufacture. One method is reducing the quantity of epihalohydrin used in synthesizing the resin, as taught in U.S. Pat. Nos. 5,171,795 and 5,714,552. Another is maintaining control over the resin manufacturing process, as taught in U.S. Pat. No. 5,017,642. Yet another is treatment of the resin with nonpolymeric amine during its manufacturing process, as taught in U.S. Pat. No. 5,614,597. Chlorohydrin residues can also be removed by adding both inorganic bases and amines after viscosity increase has taken place, as taught in U.S. Pat. No. 5,019,606; German Pat. Pub. DE 41 14 657; and European Pat. EP 0 512 423. In addition, U.S. Pat. Nos. 5,189,142, 5,239,047, and 5,364,927 teach reduced levels of organically bound chlorine in polyamine-epihalohydrin resins.
Post-synthesis treatments to reduce the quantities of AOX species in polyamine-epihalohydrin resins are also known. For example, 1,3-dichloro-2-propanol, 3-chloro-1,2-propanediol, and epichlorohydrin can all be treated with alkali to produce glycerol. Epihalohydrin and epihalohydrin hydrolyzates can be reacted with bases to form chloride ion and polyhydric alcohols, as taught in U.S. Pat. Nos. 4,975,499 and 5,019,606. U.S. Pat. No. 5,256,727 teaches reacting epihalohydrin and its hydrolysis products with dibasic phosphate salts or alkanolamines in equimolar proportions to convert the chlorinated organic compounds into non-chlorinated species, U.S. Pat. Nos. 5,470,742; 5,843,763; and 5,871,616 teach the use of microorganisms or enzymes derived from microorganisms to remove epihalohydrin and epihalohydrin hydrolysis products from wet strength compositions without reduction in wet strength effectiveness. U.S. Pat. No. 5,972,691 and WO 96/40967 teach post-synthesis treatment of wet strength compositions with an inorganic base to reduce organohalogen content after the resin has been stabilized at low pH, followed by treatment with microorganisms or enzymes. U.S. Pat. Nos. 6,056,855; 6,057,420; 6,342,580 B1, and WO 01/18093 A1 teach treatments of resins with carbon adsorbents. U.S. Pat. Nos. 5,516,885 and 6,376,578 B1, and WO 92/22601 teach removal of halogenated by-products from resins using ion exchange resins.
EP 1 13 5 427 B1 describes a process for producing epichlorohydrin-crosslinked polyamidoamines with reduced AOX content via ultrafiltration of aqueous solutions of the resin. EP 1 135 427 B1 does not disclose removal of salts or the selective removal of chloride. U.S. Pat. No. 5,009,789 teaches a method for the separation and reuse of synthetic water-soluble resins, such as urea-formaldehyde resins, melamine-formaldehyde resins, and polyamidoamine-epichlorohydrin resins, of different molecular weight produced in a polymerization zone. U.S. Pat. No. 5,009,789 teaches that the separation is carried out so that at least 5 percent by weight of the original dry resin content is separated off in the permeate. U.S. Pat. No. 6,056,967 and WO 00/67884 disclose processes for subjecting an aqueous solution of a mixture of water-soluble, amino-containing condensates or adducts having an initial molecular weight distribution to ultrafiltration through membranes, wherein the condensates or adducts are selected from a variety of resins. The Journal of Applied Polymer Science, vol. 30, pp. 4099-4111, (1985) discloses that polyamidoamine-epichlorohydrin resins can be separated into a plurality of fractions by ultrafiltration. U.S. App. Pub. No. 2001/0034406 limits its ultrafiltration process to lower molecular weights fractions of 10,000 Daltons or less and JP 2002-201267 limits its ultrafiltration process to a molecular weight range of 3,000 to 30,000 Daltons. U.S. Pat. No. 5,643,430 discloses a process for reducing the content of organic and inorganic halogen in an aqueous solution of a nitrogen-containing epihalohydrin-based resin, characterized in that the aqueous solution is subjected to an electrodialysis treatment.
Even in view of the foregoing approaches, there continues to be a need for further improvement in preparing polyamine-epihalohydrin resins, specifically a need for further improvement in processes to remove residuals such as AOX species, chloride salts and other low molecular weight species from such resins. In particular, there remains a need for a more efficient and cost effective method of removing residuals from polyamine-epihalohydrin resin-based compositions, such as wet strength agents, dry strength agents, creping adhesives, and wood product adhesives. There also remains a need for polyamine-epihalohydrin resins and polyamine-epihalohydrin resin compositions having low AOX and inorganic chloride content, as well as paper products and wood product comprising such resins.
Unless otherwise stated, all patents, patent applications, articles, textbooks and any other publications cited herein are hereby incorporated by reference in their entireties to the extent they are not inconsistent with the present disclosure. The present disclosure supersedes these incorporated patents, patent applications, articles, textbooks, and any other publications to the extent they are inconsistent with the present disclosure.